Uhlisch, D. (Corresponding author)FZJ*

1998
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Report No.: Juel-3490

Abstract: From scientific point of view as weIl as with regard to possible applications in superconducting electronics it is interesting to combine quantum effects and ballistic transport in semiconductors with the superconducting state. Due to the high mobility and long de Broglie wave length of the charge carriers, two-dimensional electron gases (2DEG) are suitable for observing the mentioned effects. Therefore, in this work basic investigations of single superconductor/2DEG contacts are presented. The high-mobility 2DEG used in this work forms at the interface of a GalnAs/InP heterostructure. As contact materials the superconductors Nb and NbN as well as the normal meta.ls Au and Co are useel. In order to prepare elirect contacts to the buried 2DEG, the heterostructure has been etcheel uncovering the 2DEG at the mesa. In this way contacts of the mentioned materials to the 2DEG have been prepared under the same optimized conditions. They are compared with respect to their normal state contact resistances. Additionally, Nb/GaInAs/2DEG contacts using GalnAs as an intermediate layer are investigated. It turns out that argon ion bombardment used in the preparation process alters the conductivity of GaInAs in a surface layer. In addition to these mainly techonlogical investigations, measurements of the normal state resistance of NbN/2DEG contacts in a magnetic field are presented. If the magnetic field is orientated perpendicular to the plane of the 2DEG, the normal state contact resistance drops dramatically with increasing magnetic field. This decrease is explained in terms of a model which assumes ballistic transport in the 2DEG. Having this model in hand, transport near the interface as well as the properties of the contact can be characterized in more detail. A new effect in the differential resistance of NbN /2DEG contacts in magnetic fields perpendicular to the plane of the 2DEG has been observed. This effect manifests itself in a splitting of the usual resistance maximum into a three-fold structure at voltages lower than the superconducting gap voltage. It qualitatively can be explained by a model which is an extension of the aforementioned model to the case of superconducting contacts. In this extension Andreev reflection as well as inelastic scattering of ballistic electrons is taken into account. The superposition of the different energy dependencies of these processes leads to the observed splitting of the subgap resistance maximum.

1. Institut für Schicht- und Ionentechnik (ISI)